1. Field of the Invention
This invention relates to a repair method and a repair welding apparatus for a structure, adapted to prevent an accident due to cracking of a structure and prolong a lifetime of a structure, and more particularly to a structure repair method and a repair welding apparatus, adapted to be applied to a structure in which stress corrosion cracking or fatigue cracking occurs.
2. Description of the Related Art
When a crack occurs in a structure, a machine and the like (which will hereinafter be referred to as a structure), for example, in a nuclear reactor, the structure may be exchanged as a whole, or repaired by attaching a reinforcing metal material to the cracked portion. Methods for repairing cracks mainly involve TIG (Tungsten Inert Gas) welding. However, TIG welding repair may be unsuitable for structure exposed to neutrons during long-term operation within a nuclear reactor. Neutron exposure may cause deterioration of the base material of a structure. If TIG welding is applied to such a base material deteriorated due to neutron irradiation, the amount of inputted heat may cause weld flaws.
Since the TIG welding involves melting a base material with an electric arc generated between the base material and a torch electrode, the arc becomes unstable when an oxide film is present on the base material. The TIG weld arc may also become unstable when an oxide film is deposited on or water enters an inner portion of an open flaw (i.e., crack). For this reason, stable welding operations cannot be performed when the base metal or the flaw to be repaired have an oxide layer or inner water.
In view of this, repair methods using laser beams have been developed in recent years. For example, Japanese Patent Publication (Laid-Open) No. 7-62893 discloses a repair method in which cutting and building-up are combined so as to deal with a flaw which causes, or has a possibility, to decrease the strength of a structure. According to this repair method, a crack removed by such repair work leaves behind a tensile residual stress on the surface of the repair weld zone due to the repair build-up welding. Therefore, there is the possibility of a crack recurring.
Japanese Patent Publication (Laid-Open) No. 7-75893 discloses a repair method for a structure, including a first step of removing a flaw-carrying region from the structure, thereby forming a recess in this portion, and filling the recess with a material, which contains at least one of the elements contained in a material constituting the structure, by build-up welding, and a second step for projecting energy into a surface portion of the above-mentioned region, thereby re-melting the surface portion of the region, followed by cooling the same surface portion at a rate at which resinous crystals of a size of between 0.1 μm and 3.0 μm are formed. This repair method attempts to desensitize a sensitized region occurring on the surface of a repaired weld zone by re-melting the surface portion of the repaired weld zone. However, the occurrence of tensile residual stress cannot be avoided due to shrinkage of the repaired weld zone during the repair welding and re-melting operations. Thus, this repair method does not eliminate the possibility that cracking will recur.
In the above-mentioned conventional repair methods, the occurrence of tensile residual stress in a repaired weld zone is unavoidable. Consequently, there is the possibility of cracks recurring, such as stress corrosion cracking, after the execution of a repair welding operation.